Karst and Sinkhole Hazards
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Karst and Sinkhole Hazards Definitions and Description Derived from a Slavic word meaning “barren, stony ground,” the term karst has been variously used to describe a landscape (terrain), a geologic setting (terrane), and a type of aquifer (or hydrology) characterized by features formed by the dissolution of soluble bedrock: “A terrain, generally underlain by limestone or dolomite, in which the topography is chiefly formed by the dissolving of rock and which may be characterized by sinkholes, sinking streams, closed depressions, subterranean drainage, and caves (Monroe, 1970). A terrane underlain by soluble rock in which the topography is chiefly formed by dissolving of rock … and may be characterized by sinkholes, sinking streams, closed depressions, subterranean drainage, and caves … (Field, 1999). “Karst is a unique hydrogeologic terrane in which the surface water and ground water regimes are highly interconnected and often constitute a single, dynamic flow system.” (Taylor and Greene, 2008). “An aquifer in which the flow of water is or can be appreciable through one or more of the following: joints, faults, bedding-plane partings, and cavities — any or all of which have been enlarged by dissolution (Quinlan and others, 1991). Karst has a wide geographic distribution, covering approximately 13 percent of Earth’s land surface, and is present on almost every continent underlain by carbonate rocks of various geologic age (Ford and Williams, 2007). In the United States, approximately 18 percent of all exposed or shallow bedrock exhibits, or has the potential for forming, karst or karst-like features (Weary and Doctor, 2014). Many geologic, topographic, and climatologic factors influence the development of karst, and not all karst features—such as sinkholes, caves, and springs—are present to the same extent or develop in the same way in every karst area. For example, although the occurrence and formation of sinkholes in Florida are often used for conceptual and comparative purposes, karst features there have formed in bedrock and under very different geologic and hydrogeologic conditions from karst features in Kentucky. Although comparing karst characteristics in the various states can be useful and informative, karst features are also locally to regionally unique, so that karst features in one state may not be completely analogous to those in another (Fig. 3-1). In Kentucky, approximately 50 to 60 percent of the state has karst or potential for karst, as indicated by maps showing areas where limestone crops out. About 38 percent of the state has at least some karst development that can be recognized on topographic maps, and 25 percent of the state is known to have well-developed karst features. Data obtained from the 2010 U.S. Census indicate that 2,894,115 people, or about 67 percent of the state’s population, live in a karst region (Cecil, 2015). Some of the larger Kentucky cities and towns located on karst are Frankfort, Louisville, Lexington, Lawrenceburg, Georgetown, Winchester, Paris, Versailles, and Nicholasville (all located in the Inner Bluegrass Region); Fort Knox, Bowling Green, KGS - 36 Elizabethtown, Munfordville, Russellville, Hopkinsville, and Princeton (in the Western Pennyroyal Region); and Somerset, Monticello, and Mount Vernon (in the Eastern Pennyroyal Region). (a) (b) Figure 3-1. Comparison of typical surface and subsurface karst features in the Inner Bluegrass (a) and Western Kentucky Pennyroyal (b) karst regions of Kentucky. Facts • Approximately 20 percent of the conterminous United States and 50 percent of the U.S. east of the Mississippi River is underlain by soluble and potentially karstic bedrock (Weary and Doctor, 2014). • Much of Kentucky‘s prime farmland is underlain by karst, as is a substantial amount of the Daniel Boone National Forest, with its important recreational and timber resources. • Caves are important karst features, providing recreation and unique ecosystems. Mammoth Cave is the longest surveyed cave in the world, with more than 400 miles of passages. Two other caves in the state are known to extend more than 30 miles, and seven mapped Kentucky caves are among the 50 longest in the United States (Gulden, 2018). • Kentucky is ranked fifth nationally among states affected by sinkhole hazards. KGS - 37 • Karst aquifers and springs supply drinking water and water used for agricultural, industrial, and recreational purposes to an estimated 20 to 25 percent of the global population (Ford and Williams, 2007). Karst aquifers and karst springs provide much of the base flow to surface streams worldwide, and the caves, aquifers, and springs present in karst areas provide critical habitat for a variety of rare, threated, or endangered aquatic and terrestrial organisms (Culver and White, 2004). • About 40 percent of groundwater used for drinking water in the U.S. comes from karst aquifers. • According to the Kentucky Division of Water, springs and wells in karst areas supply water to tens of thousands of private homes, and are also used by five public water suppliers serving more than 164,000 citizens in total: Hardin County Water District No. 1, Hardin County Water District No. 2, Georgetown Municipal, Cadiz Municipal, and Green River Valley Water District (Rob Blair, Source Water Protection Program, Kentucky Division of Water, written communication, April 9, 2018). Karst hazards that could have an impact on Kentucky’s citizens and infrastructure include sinkholes, flooding, and groundwater and surface-water contamination. Sinkholes are by far the largest and most frequently encountered karst hazards. Sinkholes Subsidence—a natural or human-induced process that results in progressive lowering of local or regional land-surface elevations—is a consequence of karst development and may occur over long-term (including geologic) and short-term timescales. Sinkholes, also called dolines in the scientific literature, are arguably the most distinctive and abundant type of subsidence-related feature (Ford and Williams, 1989). Sinkholes are physically manifested as closed and internally drained topographic depressions, of generally circular shape, that develop where soil or other overburden material subsides or collapses into subsurface voids. Sinkholes can form as a result of both natural (karst-related) processes and as a direct or indirect consequence of human activities. Human activities that can cause sinkholes include groundwater withdrawals, alteration or diversion of surface runoff, subsurface mining; subsurface erosion, piping, or compaction of unconsolidated soils or sediments along buried pipelines or beneath highways and roads; and decaying buried organic debris (e.g., tree roots, buried trash, and other debris). Sinkholes also form in nonkarst areas where leaking water or sewer pipes and other human activities create or result in subsidence, compaction, or subsurface erosion (i.e., piping) of soil, gravel, or other fill materials. Pipeline leakage and flooding affect highway roadbeds, pipelines, and other utility trenches, and often confuse nongeologists because the “sinkholes” created by these localized collapses may not be, and often are not, related in any way to karst or karst processes (Kuniansky and others, 2016). Sinkholes are classified by geologists using numerous descriptive terms depending on the types of geologic materials and processes or sequence of processes involved in their formation. Waltham and others (2005), for example, defined six classes of karst-related sinkholes. For simplicity, sinkholes may be grouped into two broad categories: subsidence and collapse. Subsidence and collapse sinkholes often occur together in the same karst area, and many sinkholes form as a combination of the two processes. Subsidence sinkholes form by the relatively slow and gradual subsurface dissolution of soluble bedrock and piping of unconsolidated cover materials (soil, alluvium) into fractures and conduits enlarged by solution in the epikarst, a zone of intensified weathering and dissolution at the soil-bedrock interface. Subsidence sinkholes in Kentucky are generally recognizable as broad, shallow, bowl-shaped depressions (Fig. 3-2a, b). These sinkholes are largely responsible for the rolling topography that characterizes much KGS - 38 of the Bluegrass and Western Pennyroyal Regions. Diameters can range from several tens to hundreds of feet, and shapes can be circular, elongate or irregular and complex. Figure 3-2. Examples of common subsidence sinkholes formed in a rural agricultural setting (a) and a suburban setting (b). Collapse sinkholes form suddenly by failure of the roof or arch of soil, bedrock, or other surface and subsurface materials located above subsurface karst voids and caves (Waltham and others, 2005). Their seemingly unpredictable occurrence makes them a hazard of particular concern in karst areas. Collapse sinkholes that form over voids in unconsolidated materials—soil, sediment, or brecciated bedrock—are common and are referred to as cover-collapse sinkholes (Currens, 2012. Cover-collapse sinkholes are typically steep-walled circular or funnel-shaped depressions (Fig. 3-3) having diameters that range from a few feet to hundreds of feet. Depths may vary, and may be somewhat dependent on the thickness of soil or unconsolidated material above karstified bedrock. The Kentucky Geological Survey began developing a catalog of case histories of cover-collapse occurrences in 1997 and has documented 354 occurrences throughout the state; an average of 24 new reports are received